Charge-exchange processes in titanium-doped sapphire crystals. I. Charge-exchange energies and titanium-bound excitons

The photoionization of Ti³⁺ in sapphire {α}-Al₂{O}₃$ was studied by means of one-step and two-step photoconductivity measurements. An absorption band of ${Ti}³⁺at 2700 was shown to be due to a{Ti}³⁺$ localized exciton. One-photon photoconductivity begins at the high-energy side of this band and thus the photoionization threshold can be located at 4.71±{}0.07 eV. The two-photon photoconductivity spectrum is nearly coincident with the excited state absorption and is shifted 0.6 eV to higher energy than the one-photon spectrum, due to the effect of the Jahn-Teller distortion of the ²E initial state. The origin of the transition due to valence band electron capture by Ti⁴⁺ is located at 4.17 eV. These electron and hole ionization energies are compared to values obtained by Born cycle calculations. Their sum is 0.5 eV less than the band gap of Al₂{O}₃$.